A method of operating a surgical system including an endoscope, a treatment tool, an overtube, and a sheathing tube. The method includes the following steps: removing the treatment tool from the sheathing tube inserted into a body cavity through a body wall in a state where the sheathing tube is covering an outer circumference of the overtube into which an insertion part of the endoscope and an insertion part of the treatment tool are inserted, while keeping a state where the sheathing tube is covering the outer circumference of the overtube; removing the endoscope from the overtube while keeping a state where the sheathing tube is covering the outer circumference of the overtube; and removing the sheathing tube from the body wall while keeping a state where the sheathing tube is covering the outer circumference of the overtube.